The United States Federal Aviation Administration (FAA) has long relied upon the eyesight of a human pilot as the primary method to avoid midair collisions even when transponders or radar systems are present. Lacking a human pilot, unmanned aircraft systems (UASs) do not have the advantage of this onboard sense-and-avoid safety feature. An increasing number of military, civilian, and commercial applications for UASs may lead to an increasingly crowded airspace.